outgoing: avoid repo.lookup() for converting revnum to nodeid

Differential Revision: https://phab.mercurial-scm.org/D3053

mercurial/hg.py

@@ -957,7 +957,7 b' def _outgoing(ui, repo, dest, opts):'
     ui.status(_('comparing with %s\n') % util.hidepassword(dest))
     revs, checkout = addbranchrevs(repo, repo, branches, opts.get('rev'))
     if revs:
-        revs = [repo.lookup(rev) for rev in scmutil.revrange(repo, revs)]
+        revs = [repo[rev].node() for rev in scmutil.revrange(repo, revs)]
 
     other = peer(repo, opts, dest)
     outgoing = discovery.findcommonoutgoing(repo, other, revs,